Wireless power transfer via electrodynamic coupling

ABSTRACT

Wireless power transmission (WPT) systems are provided. According to an embodiment, the WPT system uses one or more power transmitting coils and a receiver for electromagnetically coupled wireless power transfer. The electrodynamic receiver can be in the form of an electrodynamic transducer where a magnet is allowed to oscillate near a receiving coil to induce a voltage in the receiving coil, a piezoelectric transducer where the magnet causes a vibrating structure with a piezoelectric layer to move, an electrostatic transducer where movement of the magnet causes a capacitor plate to move, or a combination thereof. An alternating magnetic field from the transmitting coil(s) excites the magnet in the receiver into mechanical resonance. The vibrating magnet then functions similar to an energy harvester to induce voltage/current on an internal coil, piezoelectric material, or variable capacitor. Embodiments utilize magnetic coupling and electromechanical resonance for safe, spatially distributed, low-frequency power delivery to portable devices.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is a continuation application of U.S.application Ser. No. 13/989,293, filed May 23, 2013, which is a 371 U.S.national stage application of International Application No.PCT/US2011/046748, filed Aug. 5, 2011, which claims the benefit of U.S.Provisional Application Ser. No. 61/417,059 filed Nov. 24, 2010, whichare hereby incorporated by reference in their entirety, including allfigures, tables and drawings.

BACKGROUND OF THE INVENTION

Battery-powered systems are becoming increasingly ubiquitous forindustrial, personal, and medical use. Because of the finite energystorage available to battery-powered systems, battery replacement and/orcharging must be performed periodically over the lifetime of thesesystems. These maintenance procedures typically require physical contactor wire connections with the devices, which may be inconvenient,difficult, or costly in applications such as harsh-environment sensornetworks or implanted medical devices. Even where batteries can beeasily recharged, the ever-growing hunger for portable power haspresented an un-ignorable technical challenge. With the proliferation ofwireless devices, the power cable acts as a virtual umbilical cord andpresents a cumbersome hindrance to a completely unfettered wirelessworld.

Recently, numerous solutions have been proposed to address this powerproblem. Two promising options are energy harvesting and wireless powertransmission (WPT). Energy-harvesting systems convert ambient energysources, such as light, vibration, thermal, and acoustic sources, intoelectrical energy. The complete freedom from external electric powersources enables systems to be applied in a variety of environments.However, because of the typically low power densities of the ambientenergy sources and limited energy conversion efficiencies, currentenergy-harvesting systems have been targeting very low-powerapplications (1 μW-1 mW). Higher output powers are possible by usingvolumetrically larger energy harvesters, but this becomessize-prohibitive for compact applications. Another disadvantage ofenergy harvesting devices is that they are at the mercy of theenvironment. Intermittency and variability of the environmental energysource result in overdesign of the energy harvester to ensure adequatepower generation and require more complicated power electronics toregulate voltage and power flow, problems that are exacerbated inunpredictable environments.

In contrast to energy harvesting systems, WPT systems actively transferpower from a source to a receiver, providing deterministic control overthe power delivery. WPT relies on power transmission usingelectromagnetic fields, without requiring physical connections(conductive wire, optic fiber, waveguide, etc.) between the power sourceand receiver.

As used herein WPT refers to transmission over moderate distances, asopposed to “contactless power transfer,” which generally refers toshort-range power delivery across an electrically isolative barrierusing transformer cores on either side of the barrier. The most commonWPT approaches rely on either radiative electromagnetic waves ornear-field capacitive/inductive coupling.

In the radiative electromagnetic wave approach, a focused beam ofelectromagnetic energy is generated by the source and pointed toward thereceiver. For example, a laser can be used as the source, and aphotovoltaic material on the receiver can be used to convert the opticalenergy to electrical energy. One advantage of this “directed energy”approach is that the power can be concentrated in the focused beam,therefore enabling a large amount of power to be transferred in a smallarea. However, this necessitates knowledge of receiver location andmethods for active tracking if the receiver moves in space.Additionally, due the absorptive nature of this radiated energy, thetransmission path must be clear of objects, which may be difficult torealize in many applications.

In the near-field capacitive/inductive coupling approach, capacitivelyor inductively coupled WPT systems transfer power via spatiallydistributed, time-varying (yet quasi-static) electric or magneticfields.

Recent research has focused on near-field power transfer usinginductively coupled coils. The operating principle of these systems issimilar to air-core transformers. FIG. 1 illustrates a basicconfiguration of the inductively coupled WPT system, which uses twocoils 11 and 12 separated by a distance g and functions as a weaklycoupled, air-core transformer. Due to the weak mutual inductance betweenthe air-coupled coils, the operating frequency of such systems isusually in the RF range (1-100 MHz) to achieve reasonable efficiency.

One advantage of the capacitive/inductive coupling approach over theradiative electromagnetic wave approach is that power can be distributedover a large volume of space, and arrays of receivers are possible.

Accordingly, the range and transmittance of magnetic fields makesinductively coupled WPT attractive for many applications.

While the inductively coupled WPT systems have the benefit of usingmagnetic fields to pass through many materials and objects (as comparedto electric fields), there are practical limits to both power levels andefficiency, especially for powering wireless sensors.

According to Faraday's law,

$\begin{matrix}{{{V(t)} = {{- N}\frac{d\;\Phi}{d\; t}}},} & (1)\end{matrix}$the voltage V induced on the receiving coil is proportional to thenumber of coil turns N and the time-rate-change of magnetic flux Φthrough the coil. For a time-varying magnetic field B(t) in a stationarycoil, Equation (1) can be rewritten as

$\begin{matrix}{{V(t)} = {{- N}{\int\limits_{S}{{\frac{\partial B}{\partial t} \cdot d}\;{s.}}}}} & (2)\end{matrix}$For sinusoidal excitation, the voltage is proportional to (angular)frequency, peak magnetic flux density, and the coil area. Because theinduced power is proportional to the square of the voltage, maximizingthe product of the frequency, peak magnetic flux density, and the coilarea will increase the transmitted power.

However, there are strict safety limits on magnetic and electric fieldsfor RF power transmission that greatly restrict the range, efficiency,and thus application of these systems.

As explained above, since the power is proportional to the square of thevoltage, in order to deliver a certain amount of power, either thefrequency or the flux density in the receiving coil (the area of thereceiving coil is usually predetermined) needs to be sufficiently high,which may not be achievable without violating the safety limits.

Specifically, if small coils are used (small area, low number of turns),then the frequency and/or magnetic flux density must be increased inorder to increase the power transfer. However, the time-varyingelectromagnetic fields that permeate the power transmission media cancause safety hazards.

For example, in order to transfer Watts of power in an inductivelycoupled WPT system, the magnetic flux density that permeates the mediamay be on the order of 10⁻⁴ T. Such strong flux density is only safewhen the operating frequency is lower than 100 kHz. For even lowerfrequencies, the flux density safety limit is higher. For example, fluxdensity up to 10⁻³ T can be tolerated when the frequency is lower than760 Hz, and up to 0.4 T of static flux density can be tolerated bygeneral public.

The Institute of Electrical and Electronics Engineers (IEEE) and theInternational Commission on Non-Ionizing Radiation Protection (ICNIRP)place strict limits on electromagnetic field intensities. For example,ICNIRP data shows that up to 400 mT of static magnetic flux density issafe for the general public to avoid interference with magnetic stripsin credit cards or devices such as pace makers. For alternating current(ac) fields, IEEE C95.6 permits up to 1 mT for frequencies below 760 Hz.For frequencies from 760 Hz-100 kHz, IEEE C95.1 restricts the field to0.1 mT. Accordingly, safety limits and coil size put an upper limit onthe power level that can be transmitted via inductively coupled WPT.

Even if the magnetic field densities are kept in a safe regime, the lowmutual inductance between the coils generally requires the operatingfrequency to be high (usually >100 kHz), so that the mutual reactance issufficiently larger than the coil and radiation resistances. These highfrequencies create additional system limitations. First, thehigh-frequency electromagnetic fields may induce large eddy currents inany conductive materials that are present in the power transmissionpath. The power loss (and Joule heating) due to these eddy currents isproportional to the square of the frequency. In many home, medical, orindustrial applications, power transmission through the metal cases maybe required, so eddy current losses will reduce the overall efficiencyand may cause unwanted heating problems.

Power transmission efficiency is another consideration. Highefficiencies can be achieved for short-distance power transmissionswhere the resonators are strongly coupled, but the efficiency plummetsif the separation distance g is large relative to the size d₁ and d₂ ofthe coils 11 and 12 (see FIG. 1). For typical resonators having a Q of100, the efficiency drops below 50% when the distance is approximatelyg≈√{square root over (d₁d₂)}; with a Q of 1000, this distance can beextended by a factor of three. This establishes a fundamental designtradeoff between transmit distance and coil size; longer transmissiondistances require larger diameter coils or higher quality factors.

Another issue is robustness. To maximize the efficiency and range, theresonators are usually designed to have high quality factors. As aresult, the system performance is very sensitive to the resonatorparameters, since the transmitter and receiver must be preciselymatched. From the manufacturing aspect, the required tolerances in thecoil inductance and capacitance are very tight, which can be costly oreven impossible to realize. Even if initially matched or manually tuned,uncontrollable parasitic capacitances/inductances, for example due totemperature, humidity, and/or coil positions, can lead to mismatches inresonant frequency. In addition, these uncontrollable parasiticcapacitances/inductances can drastically reduce the transmit power andefficiency. To overcome this eventual mismatch, a complicated activetuner/controller is used to compensate for parameter variation. Thedesign of the tuner/controller increases the cost and complexity of thesystem.

Furthermore, tuning the resonant frequencies of the transmitter and thereceiver makes it difficult to power an array of receivers with onesingle transmitter. Since the high efficiency of the inductive WPTsystem relies on matching the resonant frequencies of the transmitterand the receivers, all of the receivers need to resonate at the samefrequency. This causes interference or cross talk between receivers.That is, the current flow in one receiver may induce significant voltagein other receivers. To mitigate this interference, additionalcomplicated power flow control circuits may be required.

Because of the various limitations on inductive WPT, application ofinductive WPT has been limited to short-range, highly specificapplications such as electric vehicles and consumer electronic chargingpads. While clever electronic control circuits have overcome some of thetuning challenges, there continue to be challenges in widespreadimplementation of inductive WPT because of the range, power, andefficiency limits of the existing inductive WPT structures.

BRIEF SUMMARY

Wireless power transfer devices and methods utilizing anelectrodynamically coupled approach to WPT are provided.

In accordance with certain embodiments of the invention, anelectrodynamically coupled WPT system is provided that actively deliverspower from a transmitter generating a low frequency (e.g., less than 10kHz), time-varying magnetic field to a receiver having anelectromechanical conversion mechanism. The electromechanical conversionmechanism can involve a permanent magnet and a receiver coil(electrodynamic conversion), a piezoelectric component (piezoelectricconversion), a variable or plate capacitor (electrostatic conversion),or a combination of conversion techniques. The influence of thetransmitter on the permanent magnet provides the WPT system its initialelectrodynamic aspect.

According to one embodiment of the invention, a wireless power transfersystem utilizing a transmitter and receiver includes a permanent magnetpositioned to oscillate in the vicinity of a receiving coil of thereceiver, providing a large flux density in the coil.

The receiving coil and the magnet can be configured in the same package,forming an electrodynamic transducer. The transmitting coil of thetransmitter is outside the package. The transmitting coil carries analternating current and can be connected to a power source. The fieldgenerated by the transmitting coil activates the motion of the magnetthrough magnetic force.

Even using fairly weak magnetic fields, significant mechanicaloscillations can be induced when the receiver magnet is excited near itsmechanical resonance (assuming an underdamped mechanical system). Thevibrating magnet presents a large time-varying flux density in thereceiving coil, much stronger than the field produced by thetransmitting coil.

Power is generated on the receiving coil in a manner similar to anelectrodynamic vibrational energy harvester, except the systemexcitation is provided by the magnetic force rather than an externalvibration. In particular, according to embodiments of the subject WPTsystem, the magnet excitation is provided by an external magnetic fieldrather than a mechanical vibration.

According to another embodiment, a wireless power transfer systemutilizing a transmitter and receiver includes a permanent magnetpositioned to oscillate at an end of a vibrating structure (e.g.,cantilever, membrane, fixed-fixed beam) formed of piezoelectricmaterial. In such an embodiment, the magnet and piezoelectric vibratingstructure can be configured in the same package, forming a piezoelectrictransducer. The transmitting coil of the transmitter is outside thepackage. The transmitting coil carries an alternating current and can beconnected to a power source. The field generated by the transmittingcoil activates the motion of the magnet through magnetic force. Power isgenerated in the piezoelectric vibrating structure in a manner similarto a piezoelectric vibrational energy harvester, except the systemexcitation is provided by the magnetic force or torque of the magnetrather than an external vibration.

According to yet another embodiment, a wireless power transfer systemutilizing a transmitter and receiver includes a permanent magnetpositioned to oscillate at an end of a vibrating structure connected toa plate of a variable capacitor, enabling electrostatic energyconversion. Here, power is generated by the variable capacitor in amanner similar to an electrostatic vibrational energy harvester, exceptthe system excitation is provided by the magnetic force or torque of themagnet rather than an external kinetic vibration.

Unlike inductively coupled WPT systems, where strong electric andmagnetic fields permeate the transmitting media, the strong magneticfield in the electrodynamically coupled system (from the magnet) islimited to the receiving region (near the magnet). This will greatlyreduce the exposure of human body to strong magnetic fields.

Due to the high magnetic field within the receiving coil (near themagnet) for embodiments utilizing a receiving coil for theelectromechanical conversion, not only can the operating frequency bereduced, but also the receiving coil size. Accordingly, certainembodiments of the invention can be operated at low-frequency operationand using a much smaller receiver as compared to inductively coupled WPTsystems.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic diagram of a conventional inductively coupledWPT system.

FIG. 2 shows a schematic diagram of an electrodynamically coupled WPTsystem for sensor arrays in accordance with an embodiment of theinvention.

FIGS. 3A and 3B illustrate torque mode and force mode receiverconfigurations in accordance with certain embodiments of the invention.

FIGS. 4A-4D show schematic representations of WPT system receivers inaccordance with certain embodiments of the invention where theelectromechanical conversion is electrodynamic (FIG. 4A), piezoelectric(FIG. 4B), electrostatic (FIG. 4C), and a combination of piezoelectricand electrodynamic (FIG. 4D).

FIG. 5 shows a schematic diagram of an electrodynamically coupled WPTsystem in accordance with an embodiment of the invention.

FIGS. 6A and 6B show perspective views of an electrodynamic receiversystem according to certain embodiments of the invention.

FIG. 7 shows an equivalent circuit model of an electrodynamicallycoupled WPT system in accordance with an embodiment of the invention.

FIG. 8 shows a plot of maximum efficiency vs. transmitting couplingstrength, when γ₂>>1 and γ₁<<γ₂, for an embodiment of the invention.

FIG. 9 shows a schematic diagram of an electrodynamically coupled WPTsystem in accordance with another embodiment of the invention.

FIG. 10 shows an equivalent reduced-order lumped-model of the WPT systemof FIG. 7 in accordance with an embodiment of the invention.

FIGS. 11A and 11B show photographs of an experimental setup illustratingthe electrodynamically coupled WPT system. FIG. 11A shows a photographof the whole system setup and FIG. 11B shows a photograph of theelectrodynamic receiver of the experimental setup.

FIG. 12 shows a streamline plot of the flux density generated by thecounter-directional coil currents of the experimental setup using 2Daxi-symmetric FEM simulation on COMSOL Multiphysics.

FIG. 13 shows a plot of open-circuit voltage amplitude vs. frequency at1 A excitation for the experimental setup.

FIG. 14 shows a plot of open-circuit voltage vs. input current amplitudeat 103 Hz excitation for the experimental setup.

FIG. 15 shows a plot of open-circuit voltage amplitude vs. magnetposition for 1 A current at 103 Hz for the experimental setup.

FIGS. 16A and 16B show photographs of a prototype WPT system accordingto an embodiment of the invention; FIG. 16A shows a photograph of thesystem setup and FIG. 16B shows a photograph of the electrodynamicreceiver of the prototype.

FIG. 17 shows a plot of power transmission vs. frequency.

FIG. 18 shows a plot of system efficiency and power transmission vs.distance (axial distance between transmit coil and receiver).

FIG. 19 shows a plot of output voltage vs. frequency with increasinginput power (200 mV, 400 mV, 1 V, and 2 V input voltage).

DETAILED DISCLOSURE

Embodiments of the invention provide wireless power transmission (WPT)systems that actively deliver power from a source to a receiver,providing deterministic control over the power transfer. Embodiments ofthe invention provide wireless power transfer devices and methodsutilizing an electrodynamically coupled approach to WPT. In accordancewith certain embodiments of the invention, an electrodynamically coupledWPT system is provided in which a current-carrying coil (transmitter)generates a low-frequency, time-varying magnetic field that induces asinusoidal mechanical forcing function on a permanent magnet residing ona receiver. The resulting magnet motion (e.g., vibration) can beconverted into electrical power via electromechanical conversion. Theelectromechanical conversion can be electrodynamic, piezoelectric,electrostatic, or a combination of one or more conversion methods.

The receiver is similar to a vibrational energy harvester, where, incertain embodiments of the invention, the mechanical excitation is aresult of an electrodynamic coupling between a power transmitter and amagnet on the receiver. For example, mechanical energy is induced by thepower transmitter in a receiver having a cantilever beam with a magnetictip mass. The power transmitter includes at least one transmitter coilapplied with an alternating current. For peak power outputs, thefrequency of the alternating current in the transmitter coil(s) ismatched with natural frequency of the receiver device to inducemechanical resonance.

In one embodiment, the subject WPT system can be implemented forwireless sensor nodes (see FIG. 2). In a specific embodiment, eachwireless sensor node can include a wireless power receiver having areceiving transducer in accordance with an embodiment of the subjectsystem. The receivers in the sensor array can be tuned to resonate atthe same or different frequencies, and the transmitter coil canbroadcast power in multiple frequency bands. Not only can power bedelivered to multiple receivers, the system can intelligently andselectively deliver power to specific receivers.

FIGS. 3A and 3B illustrate torque mode and force mode receiverconfigurations for certain embodiments having a magnet 31 attached at anend of a vibrating structure 32. The receiving mode may simply depend onthe position of the receiver with respect to the transmitter coil 33 atany given time.

FIGS. 4A-4D show schematic representations of WPT system receivers inaccordance with certain embodiments of the invention. According tocertain embodiments of the invention, the electromechanical conversioncan be electrodynamic (FIG. 4A), piezoelectric (FIG. 4B), electrostatic(FIG. 4C), or a combination of one or more conversion methods. Forexample, FIG. 4D illustrates a combination of piezoelectric andelectrodynamic.

FIG. 4A shows a schematic representation of a receiver portion of a WPTsystem according to an embodiment of the invention utilizingelectrodynamic conversion, where the magnet 41 vibrating along with thevibrating structure 42 to which the magnet is affixed as a result of theelectromagnetic fields generated by a transmitter of the WPT systeminduces current on coils 43 located in the receiver. One or more coilscan be used. In the embodiment shown in FIG. 4A two coils 43 areincluded.

FIG. 4B shows a schematic representation of a WPT system receiveraccording to an embodiment of the invention utilizing a piezoelectrictransducer for the electromechanical conversion in the receiver. In oneembodiment, the WPT system utilizing a transmitter and receiver includesa permanent magnet positioned to oscillate at an end of a vibratingstructure formed of piezoelectric material. For example, the vibratingstructure can have a piezoelectric layer 44, such as a piezoelectricpatch, on a vibrating beam 42 to which the magnet 41 is attached. In aspecific embodiment, the magnet 41 is at a tip of the beam 42. Forcertain embodiments of the piezoelectric-based electromechanicalconversion system, the magnet 41 with vibrating beam 42 having apiezoelectric layer 44 (or other piezoelectric vibrating structure) canbe configured in the same package, forming a piezoelectric transducer.As with the previously described systems, the transmitting coil (notshown) of the transmitter is outside the package. The transmitting coilcarries an alternating current and can be connected to a power source.The field generated by the transmitting coil activates the motion of themagnet 41 through magnetic force. Power is generated in thepiezoelectric layer 44 (or other piezoelectric vibrating structure) in amanner similar to a piezoelectric vibrational energy harvester, exceptthe system excitation is provided by the magnetic force or torque of themagnet 31 rather than an external vibration.

FIG. 4C shows a schematic representation of a WPT system receiveraccording to an embodiment of the invention utilizing an electrostatictransducer for the electromechanical conversion in the receiver. Forexample, a receiver utilizing the electrostatic energy conversion caninclude a permanent magnet 41 positioned to oscillate at an end of avibrating structure 42 (for example a beam or spring) and connected to aplate of a parallel plate capacitor 45 (or a variable capacitorconnected to a control circuit). Here, power is generated by theparallel plate capacitor 45 in a manner similar to an electrostaticvibrational energy harvester, except the system excitation is providedby the magnetic force or torque of the magnet 41 in the presence of thefield generated by a transmitting coil (not shown) rather than anexternal kinetic vibration.

FIG. 4D shows a schematic representation of a WPT system receiveraccording to an embodiment of the invention utilizing a hybrid ofconversion methods. For the embodiment shown in FIG. 4D, piezoelectricand electrodynamic conversion is utilized in the receiver. Of course,embodiments should not be construed as limited to the particularcombination described with respect to FIG. 4D. For example, a hybrid ofpiezoelectric, electrostatic, and electrodynamic conversion, a hybrid ofpiezoelectric and electrostatic conversion, or a hybrid of electrostaticand electrodynamic conversion can be utilized in a single receiver.Referring to FIG. 4D, both a piezoelectric layer 44 and coils 43 areused to convert the motion of the magnet 41 oscillating at an end of avibrating beam 42 to electric energy.

As explained in more detail below, the transmitter portion of thesystems can include one or more transmitting coil(s). The fieldgenerated by the transmitting coil activates the motion of the magnetthrough magnetic force. This provides the initial electrodynamiccomponent of the WPT systems. The transmitter portion can be separatedfrom the receiver portion by distances in the centimeter, meter, andkilometer range depending on application. For example, the transmitterportion can be at a fixed location and affect one or more receiverportions within a same room, within a same building, or even within asame city.

In operation, an alternating magnetic field from the transmittingcoil(s) excites the magnet in the receiver into mechanical resonance.The frequency of the signal of the alternating or time-varying magneticfield generated by the transmitter can be generated at frequencies ofless than 10 kHz. In one embodiment, the frequency of the signal can beselected to be between 120 Hz and 10 kHz. In certain embodiments, thefrequency of the transmitted signal can be selected to be one or more ofthe following: between 10 Hz and 50 Hz, less than 60 Hz, 60 Hz, morethan 60 Hz, between 70 Hz and 1 kHz, and one or more multiples of 60 Hzincluding 120 Hz, 180 Hz, and 240 Hz. Upon excitement by the magneticfield from the transmitting coil(s), the vibrating magnet functions aspart of an energy harvester—either to induce a voltage or current in areceiving coil or to simply provide a vibration or energy to apiezoelectric or electrostatic system.

FIG. 5 shows a representation of a WPT system in accordance with anembodiment of the invention. Referring to FIG. 5, the WPT systemincludes a transmitter 21 and an electrodynamic receiver. In accordancewith an embodiment of the invention, in the electrodynamic receiver, amagnet 53 is allowed to oscillate in the vicinity of the receiving coil52, providing a large flux density in the receiving coil 52. Therelative positions of the magnet and the receiving coil 52 are made suchthat a magnet 53 having a length L comes within at least L distance ofthe receiving coil 52 when moving. The magnet 53 can move such that itsnorth or south pole comes within the vicinity of the receiving coil 52.In certain embodiments, the magnet 53 can oscillate in the axialdirection of the receiving coil 52. In one such embodiment, the magnetcan be centrally positioned with respect to the receiving coil. In oneembodiment, such as shown in FIG. 5, the magnet 53 can oscillate in theaxial direction of the receiving coil 52 within at least a first windingof the receiving coil. In another embodiment, the magnet can bepositioned at a side of the receiving coil.

The magnet can be one or more permanent magnets. In certain embodiments,the magnet can be connected to a spring 54 (such as shown in FIG. 5) orcantilever in order to allow movement of the magnet 53 with respect tothe receiving coil 52.

In a further embodiment, soft magnets can be included at or near themagnet to further affect the magnetic fields. For example, the softmagnets can be arranged to guide or concentrate the magnetic fieldtoward the magnet. The soft magnets can also be used to shape themagnetic field as it approaches the magnet. In an alternate embodiment,the magnet can be or include one or more soft magnets that, whenmagnetized, induce a voltage in the receiving coil.

According to one embodiment, the receiver portion of the system caninclude the receiving coil and the magnet in a same package. Thecombined receiving coil and magnet form an electrodynamic transducer.

The transmitter portion of the system can include one or moretransmitting coil(s). The transmitter portion is outside the package ofthe electrodynamic transducer and connects to a power source to carry analternating current. The field generated by the transmitting coilactivates the motion of the magnet through magnetic force. Thetransmitter portion can be separated from the receiver portion bydistances in the centimeter, meter, and kilometer range depending onapplication. For example, the transmitter portion can be at a fixedlocation and affect one or more receiver portions within a same room,within a same building, or even within a same city.

In operation, an alternating magnetic field from the transmittingcoil(s) excites the magnet in the receiver into mechanical resonance.The vibrating magnet then functions similar to an energy harvester toinduce voltage/current on the receiving coil. Even using fairly weakmagnetic fields, significant mechanical oscillations can be induced whenthe receiver magnet is excited near its mechanical resonance (assumingan underdamped mechanical system). The vibrating magnet presents a largetime-varying flux density in the receiving coil, which is much strongerthan the field produced by the transmitting coil.

Power is generated on the receiving coil in a manner similar to anelectrodynamic vibrational energy harvester, except the systemexcitation is provided by the magnetic force rather than an externalvibration. In particular, according to certain embodiments of theinvention, the magnet excitation is provided by an external magneticfield rather than a mechanical vibration.

Unlike inductively coupled WPT systems, where strong electric andmagnetic fields permeate the transmitting media, the strong magneticfield in the electrodynamically coupled system (from the magnet) islimited to the region close to the receiving coil. This greatly reducesthe exposure of human body to strong magnetic fields.

Due to the high magnetic field experienced by the receiving coil nearthe magnet, not only can the operating frequency be reduced, but alsothe receiving coil size. This results in low-frequency operation and amuch smaller receiver. In certain embodiments, the receiver can beintegrated on a chip or substrate.

FIGS. 6A and 6B show one implementation of the subject receiver. Asshown in FIGS. 6A and 6B, a magnet can be mounted on one or morespring-like suspensions, and the receiving coil can be made withmultilevel traces on a printed circuit board. The circuit board can alsoprovide a platform for integration of other circuit components of thesystem.

In a further embodiment, power electronic circuits for power regulationare included in the system. For example, an ac/dc rectifier and dc/dcregulator are required to convert the receiver coil ac voltage into dcvoltage at desired voltage level to supply to the sensor system.

An equivalent circuit model of the WPT system is shown in FIG. 7. Asshown in FIG. 7, the model has three parts: the transmitting coil, thereceiver mechanical structure and the receiving coil. The transmittingand receiving coils are both modelled with a series R-L network (R₁ & L₁and R₂ & L₂). V_(S) and R_(L) are the source voltage and the loadresistance attached to the transmitting coil and receiving coil,respectively. The mechanical structure is modelled using amass-spring-damper system with mass m (kg), spring constant k (N/m) andviscous damping coefficient b (N·s/m). The electrodynamic couplingbetween each of the coils and the mechanical structure is modelled withtwo gyrators with gyration ratios K₁ and K₂ (V·s/m) representing thetransduction coefficients. The transduction coefficient K is shown inEquation (3).

$\begin{matrix}{{K = {\bullet{\int\limits_{l_{coil}}{{\overset{\rightharpoonup}{B} \cdot d}\;\overset{\rightharpoonup}{l}}}}},} & (3)\end{matrix}$where B is the flux density generated by the magnets and l_(coil) is thelength of the coil.

The circuit can be simplified by assuming that the operating frequencyunder consideration is sufficiently low, so that the inductance of bothtransmitting and receiving coils can be ignored. With this assumption,the maximum efficiency is obtained at the natural frequency, when theload resistance R_(L) is given by

$\begin{matrix}{{R_{L} = {\sqrt{\frac{\left( {\gamma_{1} + \gamma_{2} + 1} \right)\left( {\gamma_{2} + 1} \right)}{\gamma_{1} + 1}}R_{2}}},} & (4)\end{matrix}$where γ_(i) is the unitless coupling strength of the electrodynamiccoupling and given by

$\begin{matrix}{{\gamma_{i} = \frac{K_{i}^{2}}{R_{i}b}},{i = 1},2.} & (5)\end{matrix}$

The maximum power efficiency η_(max) is given by

$\begin{matrix}{{\eta_{\max} = \frac{\gamma_{1}\gamma_{2}\beta}{\left\lbrack {\gamma_{2} + {\left( {\beta + 1} \right)\left( {\gamma_{1} + 1} \right)}} \right\rbrack\left( {\gamma_{2} + \beta + 1} \right)}},{where}} & (6) \\{\beta = {\sqrt{\frac{\left( {\gamma_{1} + \gamma_{2} + 1} \right)\left( {\gamma_{2} + 1} \right)}{\gamma_{1} + 1}}.}} & (7)\end{matrix}$

For a well-designed WPT system, it is reasonable to assume that theelectrodynamic receiver is strongly coupled:γ₂>>1  (8)

Also, since the transmitting coil is much farther away from the magnetthan the receiving coil, the transmitting coupling strength γ₁ is muchsmaller than the receiving coupling strength γ₂:γ₁<<γ₂  (9)

Substituting Relations (8) and (9) into Equations (4) and (6), themaximum efficiency condition can be simplified to

$\begin{matrix}{{R_{L} = {\gamma_{2}\sqrt{\frac{1}{\gamma_{1} + 1}}R_{2}}},} & (10)\end{matrix}$

and the maximum efficiency is given by

$\begin{matrix}{\eta_{\max} = {\frac{\gamma_{1}\sqrt{\frac{1}{\gamma_{1} + 1}}}{\left\lbrack {1 + \sqrt{\gamma_{1} + 1}} \right\rbrack\left( {1 + \sqrt{\frac{1}{\gamma_{1} + 1}}} \right)}.}} & (11)\end{matrix}$

A plot of the maximum efficiency vs. transmitting coupling strength isshown in FIG. 8. This plot is subject to the assumptions given byRelations (8) and (9). Based on the modeling equations and the plot, itcan be concluded that high efficiency is related to the transmittingcoupling strength.

In accordance with certain embodiments of the invention, the couplingstrength can be increased and/or designed by increasing the averageradial flux density at the coil conductor; increasing the conductorvolume of the coil; using highly conductive material for coil conductor;and/or reducing the mechanical damping coefficient. All of these may beeffective means for increasing the overall WPT efficiency. Accordingly,certain embodiments of the invention utilize one or more of these meansfor increasing the overall WPT efficiency.

According to one embodiment, to increase the coupling strength, theshape and dimensions of the transmitting coil are optimized. Analyticand finite-element magnetostatic field solvers may be used to optimizethe magnetic field interactions. In certain embodiments, soft magnetsmay be used on the receiver and or transmitter to tailor the fieldpatterns. Additionally, structural enhancements and design/optimizationtools can be used to improve the electromechanical performance of thereceiver. For example, in one embodiment, opposing magnets are used tocreate a strong radial field to ensure a “strongly coupled”electrodynamic coupling on the receiver side.

In certain embodiments, the transmitter side frequency and power can beadjusted to provide particular distances from which the receiver cansuccessfully receive the signal. Advantageously, frequencies less thanRF frequencies used for certain inductively coupled WPT systems can beused.

FIG. 9 shows another embodiment utilizing an electrodynamic transducerin the receiver, where a permanent magnet 91 is mounted on a vibratingstructure, which is illustrated here in the form of a cantilever beam92. In the specific embodiment shown in FIG. 9, the permanent magnet 91is mounted on the tip of the cantilever beam 92 where both the permanentmagnet 91 and the cantilever beam 92 reside in the receiver 90.Depending on how the magnet 91 is positioned (magnetization direction)on the vibrating structure (shown in this embodiment as cantilever beam92), instead of the translational force from a magnet positioned asdescribed with respect to FIG. 5, the magnetic field generated by thetransmitting coil 95 induces a torque on the free end of the beam 92.

The resulting motion is converted into electrical power viaelectrodynamic generation on the receiver coil 93 (conventional energyharvester behavior). For peak power outputs, the frequency of thealternating current in the transmitter coil 95 is matched with naturalfrequency of the receiver device to induce mechanical resonance.

This WPT system can be represented by the simplified lumped model shownin FIG. 10. In operation, an ac voltage is supplied to the transmittercoil, and the corresponding voltage and power delivered to the receiveris measured across a resistive load. This is similar to the descriptionswith respect to the equivalent circuit model shown in FIG. 7 and,therefore, is not repeated here.

Several advantages of certain embodiments of the subjectelectrodynamically coupled systems as compared to conventionalinductively coupled systems include, but are not limited to one or moreof the following.

Safety: Inductive WPT systems tend to rely on high-frequencytime-varying magnetic fields that pose safety limits. In contrast,according to certain embodiments of the invention, power generation inthe receiving coil is made possible through the time-varying fieldgenerated by a moving magnet positioned close to a receiving coil, thevoltage induced in a piezoelectric material by the moving magnet, or theelectrostatic potential caused by the moving magnet. While strongmagnetic fields can exist inside the receiver, these fields are muchweaker outside the receiver package. A time-varying magnetic field fromthe transmitting coil is used to oscillate the magnet, but by leveragingmechanical resonance, this field can be weaker and have much lowerfrequency in comparison to the inductively coupled approach.

Efficiency: Inductive WPT demands large, closely spaced, and/orhigh-quality-factor coils to achieve high efficiency. The efficiency ofcertain embodiments of the subject system is limited by othertechnological factors such as the strength of the magnet and themechanical damping coefficient of the receiver. This provides additionalopportunities and design variables that may be manipulated to achievehigh efficiency.

Robustness: The power transfer and efficiency of inductive WPT systemsrely on precise matching of three frequencies: source frequency,resonant frequency of the transmitter and the receiver. According to anembodiment, in the subject system, the efficiency is maximized when thesource frequency matches only the mechanical natural frequency of thereceiver. No resonance matching is required between the transmitting andreceiving coils. Although the mechanical natural frequency may changeslightly over time or with environmental effects, it is reasonably easyto control the source frequency so that it tracks the maximum efficiencypoint.

Applicability: Application of inductive WPT systems is constrained bypower loss and heat generation that may occur in conductive materials inthe power transmission path. In contrast, since embodiments of thesubject system do not rely on strong, high-frequency magnetic fields inthe transmission media, the eddy current generation on conductiveobjects is much smaller and, in certain cases, negligible.

Arrays: In inductive WPT systems, interference and cross-talk(interference due to possible dynamic coupling between the coils)presents substantial challenges for simultaneously powering multiplereceivers. In certain embodiments of the subject system, the receiversin a sensor array can be tuned to resonate at different frequencies, andthe transmitter coil can broadcast power in multiple frequency bands.Not only can power be delivered to multiple receivers, the system couldintelligently and selectively deliver power to specific receivers. Thiscannot be implemented in the inductive WPT systems with high efficiencybecause the resonant frequency of the transmitter must match all thereceivers.

Embodiments of the invention utilize magnetic coupling andelectromechanical resonance for safe, spatially distributed,low-frequency power delivery to sensors, consumer electronics, orimplantable medical devices.

Certain embodiments of the invention can be used for industrialapplications such as remote sensors and portable equipment, personalapplications such as portable electronics and automobiles, and medicalapplications such as implanted medical devices and prostheses.

The following examples are illustrative of some of the methods,applications, embodiments and variants of the present invention. Theyare, of course, not to be considered in any way limitative of theinvention. Numerous changes and modifications can be made with respectto the invention.

Example: First Experimental Setup

An experiment was performed to illustrate an electrodynamic approach towireless power transfer. In the experiment, a Helmholtz coil-set wasused as the transmitting coil, and an electrodynamic energy harvesterwas used as the receiver. This experimental setup is shown in FIG. 11A.The Helmholtz coil-set consists of two 295 mm diameter, 124-turncircular coils (AWG 15) with 150 mm spacing. These coils are labeled as“power transmitting coils.” The resistance and inductance of each coilare 1.2Ω and 800 ρH, respectively. The electrodynamic receiver, alsoshown in FIG. 11B, consists of two attracting block magnets (NdFeB N50,6.4×6.4×1.6 mm³) clamped on a cantilever beam (17×7.5×0.2 mm³) at oneend. The other end of the cantilever beam is clamped by a bolt and a nut(aluminum) fixed to the mounting board (FR4 PCB). A circular coil (AWG36, outer diameter×inner diameter×height: 15 mm×11 mm×7.2 mm) is gluedto the mounting board underneath the magnets. The receiver is positionednear the transmitting coil using an aluminum holder and a wooden blockwith gratings.

The transmitting coils are supplied with counter directional ac currentgenerated by a signal analyzer (Stanford Research Systems, SR785) andamplified by a power amplifier (Techron 7540), creating a time varyinggradient field, which is illustrated by the plot shown in FIG. 12. Thecounter directional currents are used to create a large uniform fieldgradient. The current amplitude is measured with a current probe(Tetronix TCP312) and regulated by the signal analyzer. The inducedvoltage on the receiving coil is measured with the signal analyzer.

Initially the receiver is positioned midway between the two coils, butradially offset 80 mm as shown in FIG. 11A. In the plot of FIG. 12, theposition of the receiver is indicated by the dot. The excitationfrequency is swept around the natural frequency of the receiver for afixed current amplitude of 1 A. The open-circuit voltage frequencyresponse is plotted in FIG. 13. As shown in the plot of FIG. 13, a peakvoltage of ˜0.35 V is generated at ˜103 Hz, which is the naturalfrequency of the receiver. Based on the output impedance of the receiverat this frequency (˜15Ω, almost purely resistive), the maximum powerdelivery to a resistive load is estimated to be ˜1 mW, which issufficient to power sensors or other low-power electronics. Although theinput power on the transmitting coil is measured to be ˜500 mW and theefficiency is only ˜0.2% for this experimental setup, it should be notedthat improved numbers are easily obtainable in various implementationsof the invention. In particular, improving coupling strength willimprove efficiency. The improvements to coupling strength can beaccomplished by increasing the average radial flux density at the coilconductor; increasing the conductor volume of the coil; using highlyconductive material for coil conductor; and/or reducing the mechanicaldamping coefficient. Certain embodiments of the invention utilize one ormore of these means for increasing the overall WPT efficiency.

With the frequency fixed at the natural frequency (103 Hz), thetransmitting coil current amplitude is then varied from 0 to 1 A, andthe open-circuit voltage is recorded. The results of these measurementsare shown in FIG. 14. As shown by the linear fit to the plot in FIG. 14,the relationship between the open-circuit voltage and the input currentamplitude is almost linear.

Next, for comparison to an inductively coupled WPT system, the magnetson the receiver were removed, so that the system resembles aninductively coupled WPT system between the transmitting coils and thereceiver coil. With 1 A input current, the frequency was swept withinthe same range as the previous test. The results from the comparisontest (not shown) indicated that the induced open-circuit voltage is twoorders of magnitude lower than the electrodynamically coupled system. Inaddition, the estimated power for the inductively coupled WPT system is˜5000× lower. The open-circuit voltage of the inductively coupled systemincreases to a level similar to the electrodynamically coupled systemonly when the frequency is increased to greater than 10 kHz.

In the last experiment, the position dependency was investigated bymoving the receiver radially along the trace shown in FIG. 13. Theopen-circuit voltage vs. radial position was plotted and is shown inFIG. 15. The data shows the voltage increasing to a maximum of ˜0.75 Vas the receiver moves to the radial position of 110 mm. At this peakvoltage location, ˜5 mW of power is estimated, which is a 5× improvementover the prior results.

The experiments show that the electrodynamically coupled WPT approachcan be used for low-frequency applications. In addition, significantimprovements can be made to increase the power transfer efficiency.

The demonstrated electrodynamically coupled WPT system provides a safe,low-frequency and potentially small-size solution for broad range of WPTapplications. The system model reveals that by increasing thetransmitting coupling strength, the efficiency can be significantlyincreased.

Example: Second Experimental Setup

An experiment was performed to illustrate another electrodynamicapproach to wireless power transfer. The second experimental setup usinga transmitter coil and an electrodynamic receiver is shown in FIG. 16A.The electrodynamic receiver, also shown in FIG. 16B, consists of twoattracting NdFeB magnets clamped on a cantilever beam at one end. Theother end of the cantilever beam is clamped to a mounting board. Acircular coil is glued to the mounting board underneath the magnets. Thereceiver is positioned near the transmitting coil. Instead of thereceiver coil being positioned parallel to the transmitting coil and themagnet positioned for translational force as with the first experimentalsetup, the receiver coil is positioned orthogonal to the transmittingcoil and the magnet positioned for applying a torque on the cantileverbeam (e.g., rotational torque).

For the second experimental setup, the peak power efficiency improved to13%. For an input power of 6.3 mW, the system delivers 0.9 mW at 1 cmand 15 μW at 10 cm from the transmitting coil. In addition, the initialmodels/prototypes have depicted a possible power output in the range ofmicrowatts to milliwatts depending on the distance between thetransmitter and the receiver.

The maximum voltage and power is induced on the receiver at themechanical resonance of the system, which is shown as ˜45 Hz in FIG. 17.Referring to the plot of FIG. 18, as the distance between thetransmitter and the receiver increases, the efficiency and powerdelivery decreases. With increased input voltage amplitude, the systemexhibits non-linear response as shown in FIG. 19.

Although the efficiency of this approach contains limitations, powertransmission is possible through conductive media such as a metal wall,the human body, and even underwater applications. In addition, otherenergy-harvester techniques, such as the piezoelectric and electrostaticapproaches described with respect to FIGS. 10A, 10C, and 10D, can beused on the receiver while still using the low-frequency magnetic fieldsfor system excitation.

The demonstrated electrodynamically coupled WPT system provides a safe,low-frequency and potentially small-size solution for broad range of WPTapplications.

Any reference in this specification to “one embodiment,” “anembodiment,” “example embodiment,” etc., means that a particularfeature, structure, or characteristic described in connection with theembodiment is included in at least one embodiment of the invention. Theappearances of such phrases in various places in the specification arenot necessarily all referring to the same embodiment. In addition, anyelements or limitations of any invention or embodiment thereof disclosedherein can be combined with any and/or all other elements or limitations(individually or in any combination) or any other invention orembodiment thereof disclosed herein, and all such combinations arecontemplated with the scope of the invention without limitation thereto.

It should be understood that the examples and embodiments describedherein are for illustrative purposes only and that various modificationsor changes in light thereof will be suggested to persons skilled in theart and are to be included within the spirit and purview of thisapplication.

What is claimed is:
 1. A wireless power transfer system, comprising: atransmitter generating a time-varying external magnetic field; and areceiver generating electrical power under an influence of the externalmagnetic field by electromechanical conversion, wherein the receivercomprises: a magnet that oscillates via electrodynamic coupling underthe influence of the external magnetic field, and an electromechanicalconversion element coupled to the magnet, wherein the electromechanicalconversion element comprises a variable capacitor comprising plates,wherein the magnet is connected to a plate of the variable capacitor andthe oscillating movement of the magnet causes a time-varying capacitancechange in the variable capacitor thereby generating the electricalpower.
 2. The wireless power transfer system according to claim 1,wherein the oscillating movement of the magnet causes a correspondingtime-varying change in a gap of the variable capacitor.
 3. The wirelesspower transfer system according to claim 1, wherein theelectromechanical conversion element further comprises: a mechanicalflexure on which a piezoelectric material and the magnet are disposed,wherein movement of the magnet causes mechanical stress upon thepiezoelectric material; or a receiving coil, wherein movement of themagnet induces a voltage in the receiving coil.
 4. The wireless powertransfer system according to claim 3, wherein the receiver is configuredsuch that a pole direction of the magnet with respect to the externalmagnetic field remains substantially the same when the magnet moves. 5.The wireless power transfer system according to claim 1, wherein thereceiver is configured such that a pole direction of the magnet withrespect to the external magnetic field remains substantially the samewhen the magnet moves.
 6. The wireless power transfer system accordingto claim 1, wherein the magnet comprises a permanent magnet.
 7. A methodof wireless power transfer, comprising: generating and transmitting atime-varying magnetic field at a first frequency using a firsttransmitter; and generating a time-varying voltage in a receiver byallowing a magnet of the receiver to move due to an influence of amagnetic force generated by the magnetic field from the transmitter, themovement of the magnet influencing an electromechanical conversionelement in the receiver, wherein the electromechanical conversionelement comprises a variable capacitor, wherein the magnet is connectedto a plate of the variable capacitor and movement of the magnet causes atime-varying capacitance change in the variable capacitor that generatesthe time-varying voltage.
 8. The method according to claim 7, whereinthe electromechanical conversion element comprises a plurality ofmechanical flexures on which the magnet is disposed.
 9. The methodaccording to claim 7, wherein the electromechanical conversion elementfurther comprises: a mechanical flexure on which a piezoelectricmaterial and the magnet are disposed; or a receiving coil, whereinmovement of the magnet induces a voltage in the receiving coil.
 10. Themethod according to claim 9, wherein a pole direction of the magnet withrespect to the magnetic field remains substantially the same when themagnet moves.
 11. The method according to claim 7, wherein a poledirection of the magnet with respect to the magnetic field remainssubstantially the same when the magnet moves.
 12. The method accordingto claim 7, wherein the first frequency is between 120 Hz and 10 kHz.13. The method according to claim 7, further comprising: generating andtransmitting a second time-varying magnetic field at a second frequencydifferent than the first frequency using the first transmitter; andgenerating a second voltage in a second receiver by allowing a secondmagnet of the second receiver to move due to an influence of a magneticforce generated by the second magnetic field from the transmitter, themovement of the second magnet influencing a second electromechanicalconversion element in the second receiver, wherein the secondelectromechanical conversion element comprises at least one of thefollowing: a second piezoelectric material, wherein movement of thesecond magnet causes mechanical stress upon the second piezoelectricmaterial; and a second variable capacitor, wherein movement of thesecond magnet causes a capacitance change in the variable capacitor. 14.The method according to claim 13, wherein the second electromechanicalconversion element comprises at least two of the following: a secondmechanical flexure on which the second piezoelectric material and thesecond magnet are disposed; the second variable capacitor, wherein thesecond magnet is connected to the second variable capacitor; and asecond receiving coil, wherein movement of the second magnet induces avoltage in the second receiving coil, and wherein a pole direction ofthe second magnet with respect to the second magnetic field remainssubstantially the same when the second magnet moves.
 15. A receiver of awireless power transfer system, the receiver generating power under aninfluence of a time-varying external magnetic field by electromechanicalconversion, wherein the receiver comprises: a magnet attached to an endof a mechanical flexure, the magnet oscillating via electrodynamiccoupling under the influence of the time-varying external magneticfield, and an electromechanical conversion element coupled to themagnet, wherein the electromechanical conversion element comprises avariable capacitor, wherein the magnet is connected to a plate of thevariable capacitor and the oscillating movement of the magnet causes atime-varying capacitance change in the variable capacitor therebygenerating the power.
 16. The receiver according to claim 15, whereinthe electromechanical conversion element further comprises: apiezoelectric material disposed on the mechanical flexure, whereinmovement of the magnet causes mechanical stress upon the piezoelectricmaterial; or a receiving coil, wherein movement of the magnet induces avoltage in the receiving coil.
 17. The receiver according to claim 16,wherein the receiver is configured such that a pole direction of themagnet with respect to the external magnetic field remains substantiallythe same when the magnet moves.